In a prior art control device, shown in German No. GM 1,853,867, the structural elements include a plate which is rounded off on its edge and a template or pattern member in the shape of a hollow cone, the peak of which is the swivel point of the control lever. The template has two grooves opposite each other in which each limitative wall is in the shape of a circular arc in transverse cross section. Two potentiometers are mounted in two perpendicular guide planes and can be adjusted with this known control mechanism.